Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In cells utilizing a proton exchange membrane or an acid as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is a proton exchange membrane ("PEM") electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include aqueous solutions of phosphoric acid (PAFC) or potassium hydroxide (AFC) held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the low temperature performance of PEM fuel cells is superior to other fuel cells, and because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. PEM fuel cells are also superior because the PEM environment is less corrosive than the aqueous electrolytes. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention. As is well-known however, PEM cells have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing and oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations.
In operation of a fuel cell employing a PEM, the membrane is saturated with water, and the anode electrode adjacent to the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed by evaporation or entrainment into a circulating gaseous stream of oxidant, or by capillary action into and through a porous fluid transport layer adjacent to the cathode. Porous fluid transport plates may be used to supply water from a supply of coolant water to the anode electrode and remove water from the cathode electrode returning it back to the coolant water supply, and the plates thereby also serve to remove heat from the electrolyte and electrodes.
In operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode and rates at which water is removed from the cathode and at which water is supplied to the anode electrode. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into the external load circuit varies and as an operating environment such as the surrounding temperature of the cell varies. For PEM fuel cells, if insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid, which is typically hydrogen or a hydrogen rich gas, leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow. Additionally, if too much water is removed from the cathode by the gaseous stream of oxidant, the cathode may dry out so as to limit the ability of hydrogen ions to pass through the PEM, thus decreasing cell performance.
As fuel cells have been integrated into power plants developed to power transportation vehicles such as automobiles, trucks and buses, maintaining an efficient water balance within the power plant has become a greater challenge because of a variety of factors. For example, with a stationary fuel cell power plant, water lost from the plant may be replaced by water supplied to the plant from off-plant sources. With a transportation vehicle, however, to minimize weight and space requirements of a fuel cell power plant, the plant must be self-sufficient in water to be viable. Normally, the power plant exhaust passes through a condenser to remove excess water as condensed water. This water is recycled, converted into steam and fed to the fuel processing system to support the steam reforming and water gas shift hydrogen generation reactions in the fuel processor. When the amount of product water in the power plant exhaust leaving the power plant interface is equal to the amount of water that would be created by burning the reactant fuel, then the power plant is said to be operating at just the point of water self sufficiency. Neither is excess water being stored in the power plant, nor is make up water from another source required to support the fuel processor needs.
Furthermore, a fuel cell power plant may contain a well known fuel processing system for converting an organic fuel, such as methane or gasoline, into a hydrogen rich fuel for use within the fuel cell. Such a power plant requires water as a reactant along with the hydrocarbon fuel in the fuel processing system. Water self sufficiency for a fuel cell power plant containing a fuel processing system is defined as that point where the water recovered from the cell either internally through a water transport plate within the cell or externally by a water recovery condenser, or other water recovery means, is equal to the quantity of water required for the fuel processing reactions.
Although a PEM fuel cell was used in the explanation given above, the same requirements for water self sufficiency exists for any type of fuel cell that consumes an organic fuel and that contains a fuel processor such as a fuel cell power plant which incorporates a phosphoric acid fuel cell.
Accordingly, it is an object of the present invention to provide a fuel cell system which exchanges water between the power plant exhaust and inlet air streams to maintain the high levels of air humidification required for the fuel cell cathode for peak operating efficiency.
It is another object of the present invention to provide a fuel cell system which enhances the operation of the water recovery condenser to increase water recovery and maintain water self-sufficiency in arid operating environments.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.